I was thinking about a variation: use virtualbox to install Debian inside windows.
The virtual machine would have no network, and an encrypted lvm. I would share files between windows and Linux by sharing a host directory with the VM.
That way, I wouldn't have to reboot each time...
I realise this setup would be a little more vulnerable, but I guess it would still be reasonable.
Or am I missing something?
The truly paranoid will tell you what a horrible idea this is. Personally, I do it that way for my medium-term wallet. The long-term wallet is on a Raspberry Pi that I rarely turn on. My short-term wallet on my phone

It is theoretically possible to write a virus that grabs your password from your keyboard, and then use that to decrypt the lvm and steal your wallet (and your wallet password). But the encrypted lvm is a significant obstacle, the attacker would almost have to attack you personally instead of just spreading a wallet-stealing virus on the net. I would not worry, unless we are talking about a really large amount of BTC.